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SubjectRe: Killing clones
On Wed, 13 Aug 1997, Linus Torvalds wrote:

>
>
> On Thu, 14 Aug 1997, Richard Gooch wrote:
> > >
> > > About a year ago a least was a a discussion about fixing up the
> > > CLONE_PID flag so that thread id's would be encoded in the upper bits of
> > > the pid. One could have ps list only the proc info for the initial
> > > thread (or other thread if the initial has exited), and be able to see
> > > what initial process all the threads are associated with.
> >
> > It's a pity nothing ever went into the kernel. Threads under Linux is
> > still a bit clunky. Bit like IRIX threads, actually... At least we
> > don't have to contend with arenas!
>
> Indeed.
>
> My hope for the development of clone() was originally:
> - I laid the basic framework (done)

I adopted it the day you posted code on how to use it :)

> - people started using it, primarily for
> - pthreads (pretty much done)
> - asynchronous IO (nope)

I almost purely use threads for this....
[I have an asynchronous video-write thread as well for more-or-less an
X/OpenGL server]
> - interesting new uses that nobody thought about before (nope)

Logic calculations with threaded version of Lisp? Robotic simulation
using separate threads to mimic asynchronicity of design? (my brother's
working on this)

> - people started looking at what was missing and add kernel support

One thing is BADLY missing :
Efficient message queueing (into and out of the kernel)

Basic file I/O is NOT always what's wanted!
(with message queuing one can queue, say, a 16M network packet and not
expect the kernel to copy before sending... or transmitting to a different
process)

Any ideas on this one? [I've been trying to figure it out but I'm stuck
at how to queue chunks of local address space to other processes or memory
areas]

Let me guess - it's already IN the kernel, right?

(this isn't a threading issue really but it would be really handy for safe
IO between threads rather than raw shared memory)

Oh, and something that would make async IO threads easier is the ability
to redirect IOCTL's or the like.... (some kind of /dev/user-type device?)
[emulating consoles/GPM/et al would be easier] (GPM doesn't support 3D
devices or CAD pad-devices)
> The basic problem I had was that I had lots of ideas on what I wanted to
> be possible, but I was also aware that while I wanted a very specific
> basic design, I was by no means sure exactly which details need to be
> handled where.
>
> For example, the exact bits in the clone() flags field were made up by me
> not because I wanted those exact bits, but because I imagined that those
> bits might possibly make sense. For example, I am certain that the
> CLONE_VM bit makes sense, but does CLONE_FS make sense? I don't know.
> Maybe it would be better to include the current CLONE_FS information into
> the CLONE_FILES stuff?

What's attached to CLONE_FS that isn't attached to CLONE_FILES? (and vice
versa)

> Or take a look at the low 8 bits. I decided that to implement aio_xxx() on
> top of clone() we might be better off not using SIGCHLD, but instead have
> the death of the clone'd child send SIGIO directly. So now the low eight
> bits of clone_flags is the signal to be sent at exit time.

Thank you! (I need that.... some I/O threads [especially at debug time]
can suddenly exit unexpectidly... the graphic thread is especially bad)
And now that I think of it the reason I've avoided LinuxThreads is that it
DOES need SIGUSR1/2 as well as svgalib [which I'm still using <sigh>]

> I still don't know if people actually use this feature - or the feature
> that when the signal is something else than SIGCHLD, you have to use a
> special flag to "waitpid()" to get it to recognize the cloned children.
> Again, this was so that "waitpid()" wouldn't end up waiting for
> asynchronous IO.

Smart :)

> In short, I tried to make a basic framework that would fit my ideas of
> what a light-weight clone() would be good for, but at the same time I
> didn't want to set the design in stone - the intention was for it to
> develop as people found new ways of (mis)using the new cool thread
> feature.

Misuse? As in? <ideas, ideas - grin>

> > > * modifications to the linuxthreads package for testing all this out
> > > * other things that may be needed -- linuxthreads needs a little bit of
> > > help in the kernel in the few places and presumably other more general
> > > clone based threads need the same sort of stuff. For example,
> > > linuxthreads uses SIGUSR1, SIGUSR2, and it also needs a "manager thread"
> > > to handle things like stack cleanup at termination.
> >
> > None of these address the problem of killing child threads. Although
> > it's all good stuff you mention: nice to see someone thinking about
> > all this.
> > What was Linus' view on encoding IDs in the upper pid bits? It's
> > certainly good for grouping processes together, though it may be more
> > prone to introducing bugs than a simple CLONE_NO_PROC_ENTRY like I
> > suggested, by the simple rule that anything more complicated is likely
> > to have more bugs :-)
>
> Encoding the thread ID in the high bits was one of the ideas from the very
> beginning. That's what CLONE_PID is there for: the _intent_ was that
> CLONE_PID would change only the high bits, and then you could do a global
> kill (anything with the high bits zero would send a signal to _all_
> threads that shared the same low bits).
>
> Again, this is still an interesting approach. I'd like to see it done some
> day. The fact that the /proc fs makes it a bit harder is a misfeature, but
> that's actually due to bad /proc design (which used to make a lot of sense
> back when inode numbers was all we had, but we could do better these
> days).
>
> My personal favourite for /proc would be that any CLONE_PID threads would
> show up _inside_ the original parent (that's kind of the basic idea with
> CLONE_PID). So you'd have
>
> /proc/155/ "original" process (ie something that was
> created without the CLONE_PID bit)
> /proc/155/1 "1st CLONE_PID child"
> /proc/155/2 "2nd CLONE_PID child"
>
> or something like that.

That seems to be the most sensible.... can /proc organize thread groups?
[clip]
> > * new flag to clone() to either hide a process in /proc or reflect
> > that the flag was set (thinking about this more makes me think that
> > this scheme can give much the same flexibility as the above with less
> > work in the kernel and possibly a little more work in userspace procps
> > tools)
>
> See above about how I'd like this to work. With the /proc/155/1 setup, the
> old tools would only ever see the original parent, so to "ps" the threaded
> application would look like just one process.
>
> Additionally note that "kill -1 155" would send SIGHUP to _all_ the
> threads, and if you wanted to kill just one subthread you'd have to name
> it completely in 32 bits (ie "kill -1 $((0x10000+155))" would kill 155/1,
> and we'd probably make an extension to bash so that you can say just that:
> "kill -1 155/1" would do the math for you).
>
> Yes, this does imply that the first thread is special, but I don't see
> anything really wrong with that. If you don't want the first thread to be
> special, just don't use CLONE_PID - then all threads will have a full life
> of their own.

I suppose this can be done on a clone-by-clone basis :)
[being able to select a clone'd thread as a thread group controller is
handy too as I just use the initial thread to set up the environment then
spin off into the running application(s).... Largely the server is made
up of child threads. (mouse thread, keyboard thread, ....)
I suppose the 'first thread' could be the thread that calls CLONE_PID? Or
is it ONLY the initial thread?

.. this would imply that the first thread should be the manager thread,
right?

> > * new signals for LinuxThreads support (no more stealing of SIGUSR1
> > and SIGUSR2)
>
> Yes. This is separate from threads, though. We need this for RT signals
> anyway.

YES!!!!!
> > * new flag to clone() or new syscall prctl() so that when a processes'
> > parent dies, it is sent a signal
>
> Agreed. It's actually technically very easy to send a SIGPARENT (just do
> it in "forget_original_parent()", I think), and it needs another bit in
> the clone flags.

Perfect!

Now if only there was some way to signal that termination occurred due to
serious crash... (eg. GP fault).... [happens ALL the time with
videoserver - me using paranoid mmaps]
Though the 'SIGPARENT' works... Then I can trigger a device-watching
thread to reset the device when the parent crashes^H^H^H^H^H^H^Hquits...
[Having the keyboard in raw mode on seg-fault isn't the healthiest for
one's sanity... I rigged the mouse thread to reset everything if 2+
mouse-buttons are hit....]
(15minutes for fsck after every debug-step got old real fast)
oh my, long message... Sorry 'bout that... but
G'day, eh?
- Teunis


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